Patent GB-1031986-A (GB)
Title: Improvements in systems for differential transfer of powder developed electrostatic images
IPC: G03G15/16
Filed: 1963-03-22
Abstract: 1,031,986. Printing processes &c. RANK XEROX Ltd. March 22, 1963 [April 2, 1962], No. 11413/63. Heading B6C. A xerographic process in which differential information is recorded in latent electrostatic image form and is then used for the formation of a plurality of visible images or prints which may correspond to all or part of the information recorded. An electrostatic image is formed on photoconducting layer 12 in the usual way by exposing the charged layer to light from the original, and a second electrostatic image of opposite polarity is formed on the layer in register with the first using known processes for the transfer of electrostatic images. Alternatively, the second image may be formed by the induction image process in Wich layer 12 on its conductive backing 11 is overlaid by an electrode 14 comprising a highly insulating sheet 15 of polyethylene terephthalate, say, backed by transparent conductive layer 16 as of tin oxide coated as transparent base 17, say of glass. A potential is then applied across layers 11 and 16 by high voltage source 21, while activating radiation falls on the positively charged xerographic plate 10 as in Fig. 3, using a photographic negative as original. Hole-electron pairs form and, in the circuit shown, electrons 19 collect on layer 12 due to the electric field set up and form negative image areas. The resulting positive and negative image charges are simultaneously developed by contacting charged layer 12 with a surface carrying positively and negatively charged toner particles, the negative particles being both electroscopic and ferromagnetic and may comprise a ferrite dispersed in an insulating resin. Positively charged particles may comprise non-ferromagnetic pigment in a resin. Sharp image reproduction with the two-component developer is achieved by placing a conductive electrode such as roll 39 near plate 10 during development so that the electric field due to the charged plate is drawn externally above its surface. Plate 10 may be passed between conductive electrodes comprising grounded rolls 39, 50 of which roll 39 is supplied from hopper 51 with the two types of toner particles for development, the particles acquiring the correct charges due to their respective triboelectric relation with the material of roll 39. A charge of uniform polarity 13 now applied to the powder image by corona charging means and the image is exposed to light and the first and second images then have a single polarity charge but differ in that one is electroscopic and the other ferromagnetic and electroscopic. The image powder is then transferred bodily to a member 40 by the use of an electic field as from a corona generator, and an opposite field is then caused to transfer most of the powder image back to layer 12; that remaining on member 40 is permanently fixed by heating, plastic spray &c. The second, differential, image transfer from plate 10 on to member 41 is made by means of an electromagnet whose magnetic field transfers the ferromagnetic particles only to the transfer member 41 which is then fixed. This process may be carried out continuously with the apparatus in Fig. 5 having the plate 10 in the form of a drum rotatably mounted. Corona discharge 24 sensitizes the plate which is then exposed at 70, the second, induction image is formed by shaped electrode cylinder 80 or other transfer of electrostatic image process, and developer 30 is then cascaded over drum 10 from hopper 51. Unipolarity charge is then given by electrode 23. The first image transfers on to a member 40 fed between rollers 53a, 53b, and re-transfers back to drum 10, are controlled by voltage source 21. The differential transfers on to member 41 is controlled by electromagnet 25. Residual powder on drum 10 is removed by brush 60. The order of transfers may be reversed.